Embodiments of the present invention relate to a method of manufacturing a thin film battery.
A thin film battery typically comprises a substrate having one or more battery component films that include an electrolyte sandwiched between electrode films such an anode, cathode, and/or current collector films, that cooperate to store electrical charge and generate a voltage. The battery component films that are typically less than 100 microns allowing the thin film batteries to be less than about 1/100th of the thickness of conventional batteries. The battery component films are formed by processes, such as for example, physical and chemical vapor deposition (PVD or CVD), oxidation, nitridation, and electroplating.
However, conventional battery component films and substrate materials often limit the maximum levels of energy density and specific energy levels that can be obtained from the battery. The energy density level is the fully charged output energy level per unit volume of the battery. The specific energy level is the fully charged output energy level per unit weight of the battery. Conventional batteries typically achieve energy density levels of 200 to 350 Whr/L and specific energy levels of 30 to 120 Whr/L. This is because conventional substrates, such as Al2O3, SiO2, are relatively heavy and reduce the energy to weight ratio of the battery. Also, the battery component films of conventional batteries often fail to provide sufficiently high energy storage levels. It is desirable to have a thin film battery comprising component films on a substrate that provide higher energy density and specific energy levels to provide more power per unit weight or unit volume of the battery.
In one type of battery, higher specific energy levels are achieved using a thick cathode film which can have a thickness of 5 microns or more. The thick cathode films provides greater charge retention and faster charging and discharging rates. However, it is difficult to fabricate a thick cathode film on a substrate as the thick film will delaminate easily or cause surrounding battery component films to peel off. Typically, a cathode film is deposited as an amorphous or microcrystalline film in a single pass deposition process, and thereafter, crystallized by heating the film. For example, a cathode film comprising lithium cobalt oxide can be crystallized at temperatures of about 700° C. to obtain a thick, crystalline cathode film. However, the high crystallization temperatures needed to effectively crystallize the thicker cathode film, and the higher dimensional thickness of the film itself, causes high thermal stresses to arise from the differences in thermal expansion coefficients of the substrate and cathode materials. These stresses cause delamination and peeling off of the cathode film or even entire thin film battery structures formed over or under the cathode film. The relatively high crystallization temperatures also constrain the types of materials that may be used to form the other battery component films as these materials should not soften, melt, oxidize, or inter-diffuse at the cathode crystallization temperatures. Thus, conventional methods are often deficient in their ability to fabricate a thick crystalline cathode film for a thin film battery.
Delamination of the thick cathode films (or other films) can be reduced by applying an adhesion film on the substrate before the deposition of the cathode films. However, adhesion films are often electrically conducting films, such as metal films, and these films can cause short circuits in or between battery cells formed on the substrate when there is electrical contact between the metal adhesion film and overlying films. Insulating adhesion films, such as Al2O3 have also been used, however, ceramic insulating materials often have complex deposition processes and do not always provide as good adhesion to the substrate or overlying films as the metal adhesion films.
Thus it is desirable to have a thin film battery capable of providing higher energy density and specific energy levels. It is also desirable to have thicker cathode films. It is further desirable to reduce processing temperatures, such as crystallization temperatures, in the fabrication of the battery component films, and especially in the fabrication of the cathode films. It is also desirable to reduce the delamination of battery component films, such as electrode or other films and overlying structures.